Failure mechanism and supporting measures for large deformation of Tertiary deep soft rock

International Journal of Mining Science and Technology 25 (2015) 121–126

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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Failure mechanism and supporting measures for large deformation of Tertiary deep soft rock Guo Zhibiao ⇑, Wang Jiong, Zhang Yuelin State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 2 March 2014 Received in revised form 15 April 2014 Accepted 15 July 2014 Available online 4 February 2015 Keywords: Tertiary deep soft rock Failure mechanism of large deformation Constant resistance and large deformation bolt Countermeasures of constant resistance control

a b s t r a c t The Shenbei mining area in China contains typical soft rock from the Tertiary Period. As mining depths increase, deep soft rock roadways are damaged by large deformations and constantly need to be repaired to meet safety requirements, which is a great security risk. In this study, the characteristics of deformation and failure of typical roadway were analyzed, and the fundamental reason for the roadway deformation was that traditional support methods and materials cannot control the large deformation of deep soft rock. Deep soft rock support technology was developed based on constant resistance energy absorption using constant resistance large deformation bolts. The correlative deformation mechanisms of surrounding rock and bolt were analyzed to understand the principle of constant resistance energy absorption. The new technology works well on-site and provides a new method for the excavation of roadways in Tertiary deep soft rock. Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction As the diagenesis of Tertiary soft rock is poor, the diagenetic period is short and the rock strength is low. Thus, supporting problems of Tertiary deep soft rock roadways have become an increasing concern [1–3]. The Shenbei mining area in China is a typical Tertiary mining area, and the damage caused by large deformation of deep soft rock roadways of Qingshui coal mine is very serious. In this study, the failure mechanism and support measures for soft rock were analyzed. And reasonable countermeasures and support schemes were proposed on the basis of the engineering background of the transporting crossheading in south No.2 mining area of Qingshui coal mine. The complex geological structure causes great difficulty in exploration and mining. In recent years, the mining depth of Shenbei mining area has increased, and large deformation phenomena of soft rock during excavation and recovery are very prominent, such as roof subsidence, floor heave, and wall shrinkage. This phenomena affects the normal connections in the mine and production safety [4–6]. The traditional U-shaped shed support cannot adapt to the large deformation in deep soft rock during roadway excavation, and will cause instability and failure of the roadways. Constant and frequent repairs were needed to meet ⇑ Corresponding author. Tel.: +86 13910283906. E-mail address: [email protected] (Z. Guo).

safety requirements and reduce great security risk. Therefore, research is needed to develop support technology for tunneling in the deep soft rock of Shenbei mining area. In this study, laboratory experiments, theoretical analysis, and numerical simulations were performed to develop a support technology system that is suitable for the engineering characteristics of the Tertiary deep soft rock in Shenbei mining area. The proposed system is based on a new constant resistance large deformation bolt (rope) developed by Professor He. This bolt has proved to be effective on supporting the deep soft rock in Shenbei mining area. 2. Geomechanics characteristics 2.1. Formation lithology The main coal-bearing strata of Shenbei mining area are the Yang Liantun Group, and the main roadway is the transporting crossheading of south No.2 mining area with a length of 800 m, which is surrounded by argillaceous rock and buried at a depth of 552–587 m. The immediate roof of the coal seam swells with water and has a compressive strength of 1.34 MPa. The coal roadway (transporting crossheading of south No.2 mining area) has a compressive strength of 2.7 MPa. The floor of the coal seam is bauxitic mudstone, which expands into powder when encountering water and has a compressive strength of 5 MPa (Fig. 1).

http://dx.doi.org/10.1016/j.ijmst.2014.11.002 2095-2686/Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

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2.2. Geology and geostress The geologic structure in Shenbei mining area is complex, and a fold structure is mainly distributed in the mid-southern coal field [7]. According to the output state, the fracture structure is divided into two groups. The main structure in the mining field is a crack running 30°–50° from north to east, which is approximately parallel with the F1 fault on the eastern border of the mining field. And the other fracture runs from west to east with the primary break deriving from the above north-east crack. Based on analysis of the relation between the two main cuts in the tectonic field, the tectonic stress field is in the northeast direction here. The stress field distribution rules of Shenbei mining area were taken from the field measurement of typical points by using the stress relief method. Each measurement point had two principal stress directions which were close to the horizontal direction. The maximum principal stress was at an average horizontal plane angle of 4.7°, and the average maximum principal stress was 19.3 MPa. The tectonic stress field in the mining area was in the northeastern direction.

2.3. Composition and microstructure of surrounding rock In order to study the microstructure and mineral composition of the surrounding rock of roadway in the south No.2 mining area of Qingshui coal mine, rock samples were taken from the transporting gateway which includes A2 coal and mudstone, and the obtained samples were analyzed by energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Table 1 lists the test results. The surrounding rock contained a high content of minerals with strong expansibility, such as smectite and illite–smectite mixed layer. These rocks swell with water, which dramatically reduces their strength. At the same time, it produces a large expansion force, which negatively affects the stability of the roadway.

3. Deformation mechanism of roadway 3.1. Deformation and failure phenomena of roadway Seriously asymmetric large deformation failure phenomena, such as roof subsidence, wall shrinkage and floor heave, occurred on the roadway of the south No.2 mining area (Figs. 2 and 3).

Average (m) Time Name Histogram Min-max (m) Oil E33 shale

25

Lithologic description

Gray-green, oil content of 1-2%, easily broken

20-30 A1 Fossil layer E23 A2

6.7

Dark brown, interbedded 3.9-8.2 coal and partings 7.8 0.5-15

Gray, rich in plant fossils

Dark brown, joint development, conchoidal fracture, interbedded coal 18.5-21 and partings 19.7

Mudstone

6 30

E13 Tuff

20-40

Gray, soft, water expansion Gray, conchoidal fracture, dense and hard

Fig. 1. Coal stratigraphic column.

Traditional bolts have a small allowable deformation and cannot adapt to large deformations before snapping. U-shaped steel vaults generally exhibit arch bending, and the bent leg of a trapezoidal tent shows significant contraction failure. 3.2. Mechanism of roadway deformation and failure The geological engineering and supporting mechanics models for the roadway were set up by using FLAC3D finite difference software to reproduce the roadway deformation and failure process to analyze its deformation and failure mechanisms. The models provide a scientific basis for the optimization of support measures and parameters. According to engineering practice, the following two programs were proposed by taking the transporting crossheading of south No.2 mining area as the research object: Scheme A: anchor net + steel belt + anchor + shotcrete support, and Scheme B: anchor net + steel belt + shotcrete + retractable U-shaped steel support. The two models consist of tetrahedral elements and the calculated dimension is 20 m long, 40 m wide and 40 m high. The horizontal direction of the model was limited. The bottom was fixed. The top surface was the stress boundary with an applied load of 13.5 MPa to simulate the weight of the overlying + rock. Figs. 4–6 show the engineering geology and support mechanics models of the two schemes. Displacement diagrams of the roadway are shown in Fig. 7. The displacement of the original support was seriously. In Scheme A, the maximum roof subsidence reached 692 mm, the floor heave reached 634 mm, the wall shrinkage reached 1166 mm, and the left wall deformed more significant than the right wall. In Scheme B, the maximum roof subsidence reached 617 mm, the floor heave reached 514 mm, and the wall shrinkage reached 1233 mm. Scheme B provided more supporting strength than Scheme A, so it could control the floor heave to some extent. When high loads were applied in the schemes, the supports could not restrict the movements of rock, which is the typical asymmetric large deformation failure. The stress distributions of the two support schemes show that, after excavation, a vertical stress concentration area formed on both sides of the roadway, while a horizontal stress concentration area formed at the floor and roof, as shown in Figs. 8 and 9. The stress distribution of the U-shaped steel support showed that a stress concentration at the bottom of the U-shaped leads to failure of the U-shaped angle and collapse of the roadway. The U-shaped steel support was bending deformation, as shown in Fig. 10. 3.3. Force analysis of supporting body Fig. 11 shows that when a 7 t pre-tightening force is applied to the bolt and the deformation of the surrounding rock reaches a certain extent, the support system meets the maximum tensile strength value of 25 t. The surrounding rock’s deformation is still developing, and the support body cannot coordinate with the surrounding rock’s deformation. When the deformation of the surrounding rock reaches a certain degree, the anchor support will lose efficacy. The further development of the surrounding rock deformation eventually leads to instability of the roadway. In conclusion, a scaffolding support system has weaknesses such as low initial stiffness, uneven distribution of the support force, and a high late stiffness, which lead to part of the support system cannot bear a concentrated load and the support body cannot fully plays its role. Stress concentration can easily form at any points of the support structure. It is difficult for the support structure to adapt to the large deformation characteristics of the roadway.

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Table 1 Type and content of whole rock mineral. Lithology

Coal bed Roof mudstone Floor mudstone

Relative content of clay minerals (%)

Mixed-layer ratio (%)

S 52 47

I/S

I

K

I/S

10

5 5

85 43 53

40

Fig. 5. Support mechanics model of Scheme A.

Fig. 2. Asymmetric deformation of roof.

60 0 mm

Fig. 6. Support mechanics model of Scheme B. Fig. 3. Severe floor heave (in repair).

stability when the surrounding rock releases deformation energy. A constant resistance large deformation anchor rod (rope) is characterized by its ability to absorb the energy of the surrounding rock caused by deformation under a constant working resistance. 4. Constant resistance large deformation countermeasures 4.1. Working principle of constant resistance large deformation bolt

Fig. 4. Geomechanical model.

The elongation of the ordinary bolt (cable) is too low to accommodate the roadway large deformation. Large deformation of the surrounding rock in mining roadway is inevitable. However, the ordinary bolt (cable) cannot accommodate to the large deformation of the roadway, thus anchor rods (ropes) are frequently pulled, which will lead to serious consequences such as caving and collapse of the roadway roof. An applicable support material must be found to adapt to the large deformation of roadways in deep soft rock and to ensure

After excavation, the three-dimensional force equilibrium state of the original rock is destroyed, and the surrounding rock stress is readjusted. This leads to the rock mechanical properties cannot bear the stress concentration, and then the rock plastic zone or pull areas generated, which reduces the stability. Therefore, support measures must be taken to change the mechanical state of surrounding rock by improving the strength before large deformations occur [8–11]. (1) Elastic deformation stage When the deformation of surrounding rock and the energy are low, the axial force on the rod body is less than the designed constant resistance of the constant resistance large deformation bolt. The constant resistance device cannot extend, thus the constant resistance large deformation bolt relies on elastic deformation to prevent the deformation and failure of rock mass.

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-1.5452e+007 to -1.5000e+007 -1.5000e+007 to -1.2500e+007 -1.2500e+007 to -1.0000e+007 -1.0000e+007 to -7.5000e+006 -7.5000e+006 to -5.0000e+006 -5.0000e+006 to -2.5000e+006 -2.5000e+006 to 0.0000e+000 0.0000e+000 to 2.5000e+006 2.5000e+006 to 5.0000e+006 5.0000e+006 to 7.5000e+006 7.5000e+006 to 9.8346e+006 Interval = 2.5e+006

-6.9212e+001 to -6.0000e-001 -6.0000e-001 to -4.0000e-00-4.0000e-001 to -2.0000e-00-2.0000e-001 to 0.0000e+000 0.0000e+000 to 2.0000e-001 2.0000e-001 to 4.0000e-001 4.0000e-001 to 6.0000e-001 6.0000e-001 to 6.3411e-001 Interval = 2.0e-001

(a) Scheme A

(a) Scheme A

-5.1722e-001 to -5.0000e-001 -5.0000e-001 to -4.0000e-001 -4.0000e-001 to -3.0000e-001 -3.0000e-001 to -2.0000e-001 -2.0000e-001 to -1.0000e-001 -1.0000e-001 to 0.0000e+000 0.0000e+000 to 1.0000e-001 1.0000e-001 to 2.0000e-001 2.0000e-001 to 3.0000e-001 3.0000e-001 to 4.0000e-001 4.0000e-001 to 5.0000e-001 5.0000e-001 to 5.1455e-001 Interval = 1.0e-001

-1.4737e+007 to -1.4000e+007 -1.4000e+007 to -1.2000e+007 -1.2000e+007 to -1.0000e+007 -1.0000e+007 to -8.0000e+006 -8.0000e+006 to -6.0000e+006 -6.0000e+006 to -4.0000e+006 -4.0000e+006 to -2.0000e+006 -2.0000e+006 to 0.0000e+000 0.0000e+000 to 2.0000e+006 2.0000e+006 to 3.4384e+006 Interval = 2.0e+006

(b) Scheme B

(b) Scheme B Fig. 9. Horizontal stress distribution. Fig. 7. Vertical displacement distribution.

Fig. 10. U-shaped steel stress distribution of Scheme B.

(a) Scheme A

30

-2.0912e+007 to -2.0000e+007 -2.0000e+007 to -1.7500e+007 -1.7500e+007 to -1.5000e+007 -1.5000e+007 to -1.2500e+007 -1.2500e+007 to -1.0000e+007 -1.0000e+007 to -7.5000e+006 -7.5000e+006 to -5.0000e+006 -5.0000e+006 to -2.5000e+006 -2.5000e+006 to 0.0000e+000 0.0000e+000 to 2.5000e+006 2.5000e+006 to 3.0392e+006 Interval = 2.5e+006

(b) Scheme B Fig. 8. Vertical stress distribution.

Force (t)

25

26

Force Displace

22 18

20

14

15

10 10

6

5 0 900

Displace (cm)

-2.1449e+007 to -2.0000e+007 -2.0000e+007 to -1.5000e+007 -1.5000e+007 to -1.0000e+007 -1.0000e+007 to -5.0000e+006 -5.0000e+006 to 0.0000e+000 0.0000e+000 to 5.0000e+006 5.0000e+006 to 1.0000e+007 1.0000e+007 to 1.0680e+007 Interval = 5.0e+006

-1.0084e+007 to -1.0000e+007 -1.0000e+007 to -8.0000e+006 -8.0000e+006 to -6.0000e+006 -6.0000e+006 to -4.0000e+006 -4.0000e+006 to -2.0000e+006 -2.0000e+006 to 0.0000e+000 0.0000e+000 to 1.2753e+005 Interval = 2.0e+006

2 1100

1300 1500 Time step

1700

-2 1900

Fig. 11. Curves of stress and displacement over time step of support body for scheme B.

(2) Structural deformation stage As the deformation of roadway surrounding rock gradually accumulates, the axial force on the rod body will exceed the designed constant resistance. The constant resistance device elongates up to 1000 mm with the deformation of surrounding rock. The deformation energy of rock mass is absorbed by the structural distortion of the constant resistance devices. Compared with the constant resistance large deformation bolt, traditional bolts cannot

absorb the large deformation energy released by the deep soft rock. Therefore, traditional bolts cannot work when the elongation limit is exceeded [12–20]. (3) Limit deformation stage After the constant resistance large deformation bolts show both material and structural deformation, the deformation can be

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300

Wall shrinkage Roof subsidence Floor heave

U (mm)

250 200 150

125

resistance, and the energy of the surrounding rock was released. When the tension decreased to the constant value, the surrounding rock recovered its stability. Overall, the support technology can control the roadway stability in Tertiary deep soft rock, and it is feasible and reliable in Shenbei mining area.

100

6. Conclusions

50 0

12

24 36 48 60 72 84 96 108 T (day)

Fig. 12. U-T curve of new support.

released completely. When the external load reaches the designed constant value, the constant resistance device will stop slipping deformation, and surrounding rock will keep in a relatively stable state. Thus, the roadway surrounding rock is stabilized and can avoid roof falls, collapses, wall caving, and floor heaves. 4.2. Countermeasure of constant resistance large deformation bolt According to the above analysis, the support countermeasures for the roadways of the south No.2 mining area of Qingshui coal mine, coupled with the constant resistance large deformation bolt taken as the main support technology, can be formulated as follows:

The main conclusions obtained from the above analysis and research in this study are as follows: (1) According to engineering geology and damage characteristic, the process of roadway deformation and failure is reproduced to determine the deformation and failure mechanisms. (2) The characteristics of constant resistance large deformation bolt (constant resistance and absorbing energy) are analyzed. Furthermore, constant resistance large deformation bolt support scheme is proposed. (3) The constant resistance large deformation bolt support scheme is applied to the roadway of the south No.2 mining area of Qingshui coal mine, and the coupling support (constant resistance large deformation bolts + steel belt + base grouting anchor pipe) is proved to be effective in maintaining the roadway stability in Tertiary deep soft rock.

Acknowledgments (1) Use 3 m long constant resistance large deformation bolts to control the large deformation of roadway under dynamic pressure; (2) Use 6 m long constant resistance large deformation bolts to support the key parts of the roadway to strengthen the deep surrounding rock; (3) Use coupling support to control the large uneven deformation of coal and rock mass; (4) Use a base grouting anchor pipe to cut off the slip-line-field so that deformation from floor heaves of mining roadway can be controlled. 5. Application and effect analysis 5.1. Design scheme (1) Bolt is designed with the HMG-500 constant resistance large deformation bolts, with diameter of 22 mm, length of 3000 or 6000 mm, inter-row spacing of 800 mm  800 mm, and rows parallel arranged. (2) Anchor in floor is designed with 48 mm diameter seamless steel tube, which length of 4000 mm, inter-row bolt spacing of 800 mm  800 mm, and rows parallel arranged. 5.2. Effect analysis Displacement of the roadway supported by constant resistance large deformation bolts was measured to determine the typical time-displacement curve, as shown in Fig. 12. When coupling support scheme (constant resistance large deformation bolts + steel belt + base grouting anchor pipe) was used, the deformation of roadway surrounding rock was slow in the beginning. The tension of the constant resistance large deformation bolt did not reach a constant value, so the bolt could limit the deformation of the surrounding rock. When the tension of the bolt reached the constant value, the bolt slipped with the invariant

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